Influence of the Resonant Electronic Transition on the Intensity of the

Aug 24, 2009 - Preparation and Charge-Transfer Study in a Single-Walled Carbon Nanotube Functionalized with Poly(3,4-ethylenedioxythiophene). Hana Haj...
1 downloads 3 Views 467KB Size
Download PDF
16408

J. Phys. Chem. C 2009, 113, 16408–16413

Influence of the Resonant Electronic Transition on the Intensity of the Raman Radial Breathing Mode of Single Walled Carbon Nanotubes during Electrochemical Charging Martin Kalbac* and Ladislav Kavan J. HeyroVsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, V.V.i., DolejsˇkoVa 3, CZ-18223 Prague 8, Czech Republic ReceiVed: June 5, 2009; ReVised Manuscript ReceiVed: August 11, 2009

In situ Raman spectroelectrochemistry has been used to probe the E11S and E22S electronic transitions of the semiconducting single walled carbon nanotube tube (6,5). These two transitions were investigated through the intensities of the radial breathing mode by using 1.16 and 2.18 eV laser excitation energies, respectively. The bleaching behavior of the (6,5) tube is similar for both laser excitation energies if the magnitude of the applied electrode potential is smaller than about (0.5 V. At potentials outside the (0.5 V window, the intensity/ potential profile is steeper for the 1.16 eV laser excitation. But a significant bleaching of the Raman signal is observed also for the 2.18 eV laser excitation energy even at mild potentials. This behavior shows that the filling of the E11S electronic state has a strong impact also on the E22S electronic transition. Hence, we suggest that a broadening of the resonance profile of E22S is a major reason for the change of the resonance enhancement of the Raman spectra before the Fermi level reaches the energy of E22S transition. Introduction The electronic structure of single walled carbon nanotubes (SWCNTs) is attracting attention due to prospective application in nanoelectronic devices. The ability to controll the electronic properties of carbon nanostructures is a crucial factor for advances in the field. It has been shown recently that electrostatic gating or electrochemical charging is a precise tool to control the electronic structure of carbon nanostructures.1-11 Raman spectroscopy is a convenient method for studying the effects of doping and electronic structure of SWCNTs. This is because Raman spectra are sensitive to the population of the electronic states.9 Important features of the SWCNTs in Raman spectra are the radial breathing mode (RBM), tangential modes (TG), defect induced D mode, and high frequency G′ mode. The RBM frequency (ωRBM) scales with the tube diameter (dt) according to:

ωRBM)A/dt + B

(1)

where A ) 217.8 cm-1 · nm and B ) 15.7 cm-1.12 Hence, the diameter of the studied nanotubes can be easily determined from the Raman spectra. For small diameter tubes the combination of the RBM mode frequency and the resonance Raman profile provides sufficient information for the (n, m) assignment of the particular SWCNT. (The resonance Raman profile is a dependence of the Raman intensity on the excitation laser energy. The maximum of this profile approximately corresponds to the energy of the optical transition into the Van Hove singularity, which is employed in the resonance enhancement of the particular SWCNT.) The tangential displacement modes between 1500 and 1600 cm-1 exhibit only weak diameter dependence. Therefore in the case of nanotube bundles, containing a mixture of differently sized tubes in various interactions, it is very difficult to perform a detailed analysis of the TG band. * To whom correspondence should be addressed. Phone: 420 2 6605 3804. Fax: 420 2 8658 2307. E-mail: [email protected].

Nevertheless, the line shape of the lower frequency part (G-) of the TG mode is characteristic for metallic and semiconducting nanotubes, respectively. While for semiconducting nanotubes the G- is narrow and corresponds to the diameter-dependent TO mode, and for metallic tubes the G- is significantly broadened as a consequence of Kohn anomaly and it is attributed to LO mode. The Kohn anomaly effect is reduced or completely absent if the Fermi level shifts from its position in the pristine state as a result of charge-transfer doping. Hence, for metallic tubes the charging causes narrowing and upshift of the G- mode. These effects have been studied recently in detail.3,8,13 To follow the effect of charge on the electronic structure of carbon nanotubes, the RBM band is the most convenient. This is because the Raman resonance window of the RBM band is narrow; hence the RBM intensities should be very sensitive to the change of the electronic structure. Furthermore, due to the diameter dependence of the RBM band frequency (eq 1) it is possible to follow the changes in electronic structure for tubes with different diameter. The effects of charging on the RBM are mainly represented by the change of the intensity of the Raman signal. It has been suggested that electrochemical charging shifts the Fermi level. When the Fermi level reaches the Van Hove singularity, the latter is depleted/filled from/by electrons, hence the electronic transitions are hindered if they involve this particular singularity. When this electronic transition is employed in the resonance Raman enhancement, a strong attenuation of the Raman signal is expected. This explanation qualitatively rationalizes the observed effects. However, a more detailed analysis of the experimental results shows that the bleaching of the Raman signal is observed already before the Fermi level reaches the singularity that is involved in resonance enhancement.2 Recently, it has been suggested that adding/removing a charge carrier into/from any electronic state has a significant impact on the whole electronic structure.2 As a result of the change of electronic structure the resonant Raman spectra change even if the Fermi level does not reach the singularity that is involved in the resonance enhancement. This also results in a dramatic

10.1021/jp905312b CCC: $40.75  2009 American Chemical Society Published on Web 08/24/2009

Semiconducting Single Walled Carbon Nanotubes

J. Phys. Chem. C, Vol. 113, No. 37, 2009 16409

Figure 1. Raman spectra of CoMoCAT SWCNT in 0.2 M LiClO4 in acetonitrile at OCP (open circuit potential) excited by 1.16 and 2.18 eV, respectively. The spectra are normalized against the intensity of the RBM mode of tube (6, 5) which is marked by an arrow. The spectra in the left chart are further zoomed by a factor of 3.1.

difference in the Raman spectra of the metallic and semiconducting tubes. While for the metallic tubes the RBM mode starts to bleach when the Fermi level is shifted only slightly from EF ) 0, for semiconducting SWCNTs there is an offset in the bleaching behavior and the Raman signal changes significantly only after the Fermi level reaches the energy of the first Van Hove singularity (above/below about (0.3 V). The effect of doping on electronic transitions was also studied by optical absorption spectroscopy.14,15 Although the bleaching of absorption bands is clearly detectable, a detailed interpretation of the results is difficult. For a sample containing different and bundled nanotubes, the vis-NIR spectroscopy usually does not allow accurate assignment of the particular (n, m) tubes. Furthermore, in the case of polydisperse samples it is hard to observe a broadening of the Van Hove singularities in optical absorption spectroscopy. In contrast to the optical absorption, Raman spectroscopy is very sensitive to a broadening of the Van Hove singularities16 since the signal intensity is dependent on the density of electronic states. In the previous experiments the electrochemically driven bleaching of the Raman intensities has been studied mostly for high energy transitions (E22S, E33S, and E44S) in semiconducting tubes and for E11M in metallic tubes.2 Furthermore, the abovediscussed conclusions were based on the comparison of the filling/depleting of the Van Hove singularities in different SWCNTs. Nevertheless, it would be desirable to compare the effect of the electrode potential on the bleaching of the Raman spectra if different electronic transitions are in resonance with laser excitation energies for the same SWCNT. This experiment would provide unique information about the importance of the filling of Van Hove singularities in comparison to the doping induced broadening of Van Hove singularities. For most of the Raman measurements of metallic carbon nanotubes, the E11M transition is active for resonance enhancement. This is because the energy of E22M transition is relatively high, hence this transition is not available for the usual Raman experiments. In contrast, for semiconducting tubes, several transitions are available for Raman measurements. Therefore it is in principle possible for the same semiconducting nanotube to compare the effect of doping on the Raman spectra if different Van Hove singularities are involved in the resonance effect. The most convenient approach would be to compare the

resonance effects associated with the E11S and E22S transitions, respectively. The higher transitions are less appropriate for such comparison, because they exhibit a broadened resonance profile.17 Furthermore, at higher transitions energies (E33S or E44S) many tubes are in resonance, which complicates the assignment of the (n, m) indices from the Raman spectra even for relatively thin diameter tubes. To evaluate the effects of charge carriers on singularities which are in resonance with the laser excitation energy, the experimental data on tubes which can be excited both through E11S and E22S are desirable. However, there are only limited possibilities to probe the E11S because either extremely narrow tubes or low excitation energies (below ca. 1 eV) are required. One of the few exceptions which are experimentally accessible is the tube (6, 5) demonstrated here. For this nanotube, we probed the E11S electronic transition using laser excitation of 1.16 eV and we compared the results with data on the same tube but for the laser excitation at 2.18 eV, which is in resonance with the E22S transition. On the basis of our experimental results we were able to evaluate the importance of two fundamental spectroelectrochemical effects, viz., broadening vs. filling/ depletion of Van Hove singularities which are responsible for the bleaching of Raman spectra of charged nanotubes. Furthermore, we show that the bleaching of the Raman spectra starts when the first Van Hove singularity is filled/depleted independently on a transition that is in resonance with the laser excitation energy. The bleaching behavior was found to be faster when the E11S electronic transition was in resonance with the laser excitation energy. Nevertheless, a significant bleaching of the Raman signal was observed also for spectra excited by 2.18 eV laser radiation, which is in resonance with the E22S transition. Results and Discussion Figure 1 shows the Raman spectra of the SWCNT sample (CoMoCAT) excited by 1.16 and 2.18 eV laser excitation energies, respectively. The RBM region of the spectra excited by 2.18 eV is dominated by the signal at 310 cm-1 (marked by an arrow on Figure 1). This band is assigned to the semiconducting tube (6, 5). The assignment of (n, m) of the tubes with narrow diameters based on the Raman spectrum is facilitated since there are only a few candidates for the particular diameter and usually only one satisfies the resonance condition for the

16410

J. Phys. Chem. C, Vol. 113, No. 37, 2009

given laser. For CoMoCAT samples it is furthermore known that they are dominated by the tube (6, 5) (more than 50%, see ref 18). The spectrum in Figure 1 (bottom curve) is excited by the 1.16 eV laser and it exhibits only two intensive RBM bands. One of them is located at 310 cm-1. This band was also assigned to tube (6, 5), but now it is in resonance with the E11S transition. Previous experiments suggested a value of 1.27 eV for the E11S transition of the tube (6, 5),19 which is slightly higher than the laser excitation energy used (1.16 eV) in our study here. However, the value of 1.27 eV was obtained on individualized tubes which were wrapped by a surfactant. Here in this study we measured bundled sample without a surfactant. The bundling generally causes a red shift of the resonance maxima by about 30-70 meV.20 Hence, the (6, 5) tube in our bundled sample is, presumably, close to a resonance with the 1.16 eV laser. Furthermore, the (6, 5) tube is dominating the sample and the resonance enhancement is strong for the E11S transition.21 Therefore the (6, 5) tube can give a reasonable signal even if the laser excitation energy does not perfectly match the energy of the E11S transition. Finally, there is no other candidate for the frequency of the RBM band at 310 cm-1. One may note that the value of the RBM band frequency may be influenced also by environmental effects. However, a similar effect would be expected also for the 2.18 eV laser energy excitation. Since the RBM position at the latter excitation is also 310 cm-1, it ensures that in the case of our sample the environmental effects do not have a significant influence on the RBM mode frequency. The second most intensive RBM band (at 330 cm-1) in the spectra excited by the 1.16 eV laser excitation energy can be assigned to the tube (7, 3).17 Despite the E22S ) 2.11 eV value of the tube (7, 3) not being far from the laser excitation energy 2.18 eV, the effect of bundling shifts the resonance maximum to even lower energies and thus the signal of the tube is not detectable through the E22S transition at our experimental conditions. On the other hand, the bands at 290, 338, and 422 cm-1 in the spectra excited by the 2.18 eV laser excitation energy, which were tentatively assigned to (9, 2), (6, 4), and (5, 3) tubes, respectively, do not appear in the spectra excited by the 1.16 eV laser excitation energy and therefore they are not analyzed in this study. Obviously, the tube (6, 5) is the only candidate, accessible through the E11S and E22S transitions in our setup. The tangential mode region exhibits typical features of semiconducting tubes for both probed laser excitation energies. There is the G+ mode at about 1590 cm-1 and two narrow Gmodes at 1525 and 1545 cm-1. The (n, m) assignment of the TG bands is difficult for nanotube bundles since there is only a weak diameter dependence of these bands. Therefore we further focus only on the RBM region, where the bands can be unambiguously assigned. Figures 2 and 3 show Raman spectra of the RBM region excited by 2.18 and 1.16 eV laser excitation energies, respectively, at different electrode potentials. The negative charging corresponds to adding the electrons to the SWCNT while the positive value of the potential corresponds to removing electrons from the electronic states of SWCNTs. The potential is referred to the Ag/Ag+ pseudoreference electrode. From a different point of view the reference electrode also can be considered as a gate electrode. In this case a working electrode would be considered as being at 0 V and the value of the gating potential (applied to the reference electrode) is given. For the latter notation a positive value of electrode potential would correspond to electron injection while a negative value corresponds to electron withdrawal.

Kalbac and Kavan

Figure 2. The Raman spectra in the RBM region at electrode potentials from -1.5 to 1.5 V. The electrode potential step between two adjacent spectra is 0.1 V. The spectra are excited by 2.18 eV laser excitation energy. The scale is identical for all the spectra, but the spectra are offset for clarity. The spectrum at 0 V, which is nearly the undoped state, is highlighted by a bold curve.

Figure 3. The Raman spectra in the RBM region at electrode potentials from -1.5 to 1.5 V. The electrode potential step between two adjacent spectra is 0.1 V. The spectra are excited by 1.16 eV laser excitation energy. The scale is identical for all the spectra but the spectra are offset for clarity. The spectrum at 0 V, which is nearly the undoped state, is highlighted by a bold curve.

It is clearly demonstrated that by increasing the magnitude of the electrode potential the Raman intensity of all bands in the RBM region is decreased. This is valid for both 2.18 and 1.16 eV laser excitations (Figures 2 and 3). A similar bleaching was observed also for the TG mode (data not shown). However, in case of the TG mode, the signal is composed from a contribution of several tubes, which makes the interpretation of the results difficult.

Semiconducting Single Walled Carbon Nanotubes

J. Phys. Chem. C, Vol. 113, No. 37, 2009 16411

Figure 4. The normalized Raman intensity vs. electrode potential for the RBM band at 310 cm-1. The values of intensities are taken from Figures 2 and 3. Squares and circles correspond to spectra excited by 2.18 and 1.16 eV laser excitation energies, respectively. The intensities at all electrode potentials are normalized to the intensity at 0 V for the appropriate laser excitation. The solid lines serve as a guide for the eye.

The bleaching behavior has been traditionally explained by a model based on the filling/depleting of the Van Hove singularities, which are involved in optical transition causing resonance enhancement at the given laser excitation energy. The change of electrode potential causes a shift of the Fermi level.9 If the Fermi level reaches a particular Van Hove singularity, it is filled by electrons or depleted from electrons for negative or positive charging, respectively. The optical transitions involving the filled/depleted Van Hove singularities are blocked. Consequently, if this Van Hove singularity is in resonance with the used laser excitation energy, the resonance enhancement is removed and the Raman signal is bleached. But more precise analysis of the bleaching behavior showed that the Raman signal is bleached before the Fermi level reaches the singularity involved in the resonance enhancement. Furthermore, the bleaching behavior appeared to be different for semiconducting and metallic tubes.2 It has been suggested that filling/depleting of any electronic states leads to the change of a whole electronic structure of SWCNTs and thus the bleaching may be observed before the Van Hove singularity (which is employed in resonance enhancement) is filled/depleted.2 To analyze this effect in more detail, we plotted the RBM intensity vs. electrode potential for the band at 310 cm-1 in Figure 4. Two curves represent the spectra excited by 2.18 and 1.16 eV laser excitation energies, respectively. For clarity, intensities are normalized to the value at 0 V for the particular laser excitation energy. The plot excited by using Elaser ) 1.16 eV (circles) shows the intensity/potential profiles for the case, where the E11S transition is in resonance with laser excitation energy. The bleaching of the Raman intensity begins to be fast at about (0.5 V. The laser excitation energy is 1.16 eV and thus (0.58 V is necessary to fill the first Van Hove singularity (E11S). In fact slightly higher potentials are expected to bleach the optical transition, since the resonance effect includes exciton but the level of VHs is not affected by exciton for doping. There is also a small asymmetry for positive and negative doping. The positive potentials lead to a slightly faster bleaching of IRBM than in the case of negative electrode potentials. This is in agreement with theoretical calculations showing the

asymmetric character of Van Hove singularities.18 Furthermore, for the 1.16 eV laser excitation energy is the maximum IRBM shifted toward positive values (to about 0.2 V). This shift is not found in the case of the 2.18 eV laser excitation energies. The change of IRBM at potentials when the E11S is not attenuated by filling/depletion is very small (about 2% per 0.1 V) and can be attributed to the change of the nanotube environment. Due to the charging of the nanotube film the electrolyte counterions are reorganized to compensate the applied charge. It is known that exciton binding energy is sensitive to the nanotube environment and therefore the resonance maximum may be changed. Since the 1.16 eV laser excitation energy may be slightly lower than the value of E11S transitions the change of the nanotube environment can shift E11S and thus the laser energy can be better matched. This would lead to an increase of the Raman intensity. For the E22S transition, the laser 2.18 eV excitation energy fulfills the resonance condition better, thus the change of the E22S transition energy would rather lead to a bleaching of the signal. Furthermore, the resonance profile for the E22S transition is broader,17 hence a possible small shift of the resonance maxima is influencing the Raman intensity less significantly. The second curve (squares) in Figure 4 shows a plot of IRBM vs. potential for the same tube (6, 5) but the spectra are excited by using Elaser ) 2.18 eV. Here, the laser excitation of 2.18 eV is in resonance with E22S of the tube (6, 5). Therefore in terms of the simplest model of energy level shifts, the potentials exceeding (1.09 V must be applied to fill/deplete the states involved in the E22S transition. Moreover, a higher magnitude of the electrode potential would be required, due to the effect of exciton and because the doping efficiency at these magnitudes of potential is between 50% and 70%.22 (The doping efficiency gives the relationship between the applied electrode potential and the shift of the Fermi level. For example, the doping efficiency of 50% means that the potential of 1 V would shift the Fermi level by 0.5 eV.) However, the significant change of the Raman intensity is observed already when the electrode potential exceeds the values of about 0.5/-0.5 V. Furthermore the profile IRBM vs. potential is not very different from that measured for E11S resonance (Elaser ) 1.16 eV). Hence, Figure

16412

J. Phys. Chem. C, Vol. 113, No. 37, 2009

4 confirms that the bleaching can be observed before the Van Hove singularity, which is employed as the resonance effect is filled/depleted. The bleaching of IRBM exhibits an offset of about 0.5 V for both probed laser excitation energies. At this potential the first Van Hove singularity starts to be filled/depleted. When the magnitude of electrode potential is increased above (0.5 V, the behavior of IRBM in dependence on electrode potential is non-ideal. The IRBM for the 2.18 eV laser excitation energy should not be changed until a potential of about 1.1 V is reached, but IRBM is decreased already when the magnitude of the electrode potential is increased above (0.5 V. On the other hand, for the 1.16 eV laser, IRBM should drop to zero at the moment when electrode potential reaches the first Van Hove singularity (E11S); however, the signal can be traced even at a magnitude of the electrode potential above (1.3 V. This behavior may be explained by the broadening of the Raman resonance window induced by charging.2,16 In case of 2.18 eV laser excitation energy, the filing/depleting of the first Van Hove singularities (E11S) causes the broadening of the resonance window for the E22S transition (which is probed by the 2.18 eV laser) and the signal intensity (IRBM) is decreased. For the 1.16 eV laser excitation, only the complete filling/ depleting of the singularity should result in total vanishing of the signal. However, the resonance window also can be broadened by partial filling/depleting of the electronic states. Therefore the doping of E11S may induce broadening of E11S itself, which would result in less effective bleaching of the Raman signal, and thus it can be observed even at high values of electrode potentials. It should be noted that for the semiconducting SWCNT the laser must be in resonance with the excitonic/optical transition. However, recently we performed experiments on individual metallic tubes and we also observed a broadening of the resonance Raman profile induced by doping.16 Since the excitonic effects are not present or they are much weaker in metallic tubes, we believe the role of exciton is not dominant. Consequently the bleaching of the Raman signal can be attributed mainly to the broadening of Van Hove singularities. It is evident that the broadening of the resonance Raman profile induced by charging has a different effect on the bleaching behavior of IRBM, which involves E11S and E22S electronic transitions, respectively. While for E11S it causes less effective bleaching, for E22S it leads to the bleaching even before the Fermi level reaches the energy of this transition. Therefore the Raman intensity/electrode potential profiles are becoming more similar for the 1.16 and 2.18 eV laser excitation energies despite their probing different electronic transitions. Nevertheless, the profiles still exhibit significant differences. Figure 5 shows a plot of the normalized IRBM for the 1.16 eV laser excitation energy divided by the normalized IRBM for the 2.18 eV laser excitation energy at different electrode potentials. It is clear that before the Fermi level reaches the energy of the first electronic transition (E11S) there is almost no difference between the bleaching behavior of the spectra enhanced through E11S or E22S, that is, excited by the 1.16 or 2.18 eV laser excitation energies. At the magnitude of electrode potentials between (0.5 and (1.0 V the bleaching of the IRBM is significantly faster for the 1.16 eV laser excitation energy referenced to that for the 2.18 eV laser energy excitation. In this interval (from (0.5 to (1 V) the bleaching of the Raman spectra excited by 1.16 eV is caused by filling of the E11S, but the bleaching of the Raman spectra excited by 2.18 eV is caused by broadening of the resonance Raman profile which includes the E22S transition.

Kalbac and Kavan

Figure 5. The normalized IRBM for the 1.16 eV laser excitation energy divided by the normalized IRBM for the 2.18 eV laser excitation energy at different electrode potentials.

Obviously, the filling of the Van Hove singularities bleaches the Raman spectra more efficiently than the broadening of resonance Raman profile. At potentials outside the (1.0 V window the bleaching is similar for both laser excitation energies. At this potential range the E22S transition also starts to be filled/depleted with electrons and consequently the bleaching is again similar for spectra excited by 1.16 and 2.18 eV laser excitation energies, respectively. A comparison of the bleaching behavior of other tubes in the spectra excited by 1.16 and 2.18 eV shows that we can observe a similar trend as in the case of tube (6, 5), which was analyzed in this study in detail. For example, the bleaching behavior of the band at 330 cm-1 (Figure 3) is similar to that of the band of (6, 5) in the same figure. On the other hand, it is very different from that of the (6, 5) tube in Figure 2, which was excited by 2.18 eV laser excitation energy. It is very likely that also for other tubes in the sample, the lower laser excitation energy (1.16 eV) would resonate with E11S while for higher energy (2.18 eV) would resonate with E22S of these tubes. However, in the latter case the effect of different transitions cannot be extracted from other parameters, for example, the tube chirality and/or diameter.23 Generally, the situation can be even more complex and the diameter dependence can be further modulated by a resonance condition.24 This excludes a detailed analysis of such results. It also should be noted that the effect of bundling may cause some deviations from an ideal behavior. For example, the bleaching of the Raman bands is not complete within the probed potential range. This may be a consequence of the difference between the potential at the surface of the bundle and that in the bulk of the bundle. Also the limited penetration of the compensating ions into the inner space of bundles may lead to the less effective doping. Nevertheless, these effects are not dependent on laser excitation energy and therefore they should be the same for both the 1.16 and 2.18 eV laser excitations. Conclusions In situ Raman spectroelectrochemistry of the semiconducting tube (6, 5) has been used to probe E11S and E22S electronic transitions, using 1.16 and 2.18 eV laser excitation energies, respectively. Before the electrode potential reaches the value of about (0.5 V, the bleaching behavior of the IRBM of the (6, 5) tube is similar for both laser excitation energies. At magnitudes of the electrode potential above (0.5 V the slope of the bleaching of the Raman signal vs. electrode potential is higher for the 1.16 eV than for the 2.18 eV laser excitation energy. However, a significant bleaching of the IRBM is observed also for the 2.18 eV laser excitation energy, when the potential

Semiconducting Single Walled Carbon Nanotubes exceeds the value of about (0.5 V. This behavior shows that the filling/depletion of E11S electronic states has a strong impact on a whole electronic structure, including the E22S electronic transitions. This is a very important result since it shows that the mechanism of the bleaching of the resonance Raman spectra is different for different electronic transitions. While for the E11S the bleaching is mainly related to the blocking of electronic transitions due to a filling/depleting of the particular electronic states, for the E22S transition the bleaching of the Raman signal reflects the changes of electronic structure (like shifts and broadening of electronic bands) due to filling/depleting of the E11S electronic states. Experimental Section The CoMoCAT SWCNTs were purchased from SWeNT (USA). The electrodes for in situ spectroelectrochemical studies were fabricated by the evaporation of the sonicated ethanolic slurry of SWCNTs on Pt electrodes. The film was outgassed at 80 °C under vacuum and then the electrode was mounted in the Raman spectroelectrochemical cell. The spectroelectrochemical cell was airtight, had a single compartment, and was equipped with a glass optical window for spectroscopic measurements. The cell was assembled in a glovebox (M. Braun); the glovebox atmosphere was Ar containing